Is there a noticable difference in loft insulation (rockwool rolls) in
insulating greater than a 4" depth?

Yes. The law of diminishing returns says that 350mm is the best
environmentally speaking. Any more and the environmental cost of
manufacturing and transporting the insulant is higher than the energy saved.
100mm is definitely on the low side. 200mm would be better. The difference
between 200mm and 350mm isn't that great.

Christian.